Unemployment solved: 20-hour work week

While watching yet another presidential candidate promise how he would stimulate the economy to create more jobs, I found myself wondering why nobody is talking about an obvious solution: restricting the supply of labor by cutting the working hours.
 
Given the current productivity levels and 5 billion people of working age on the planet, it does not seem possible to give everyone a 40-hours/week job doing something useful.  There is a precedent for reducing working hours.  We went from 70 or more to 40 a while ago.  We also eliminated child labor.  Was it a coincidence that real wages went up and the middle class got created since then? 

Why is 20-hour work week a bad idea, or a good one?

I’d phase it in quite slowly. Maybe cut back 2 hours every year over ten years.  In many countries there are already laws mandating 40-hour work week.  They would just be updated to reflect the new hours.  Countries that don’t have them would be “persuaded” by WTO, IMF, World Bank, trade unions, etc. to adopt and follow them.  

The phase-in would minimize the disruption to the economy by providing time for re-training etc.

There might need to be some kind of debt adjustment/forgiveness. For example, if a family has an existing mortgage that assumes certain level of income, the debt could be reduced as well.